1. Field of the Invention
The present invention relates to a method for maximizing the directivity and adaptation of directive microwave couplers, operating, for example, at frequencies between a few hundreds of Megahertz and several tens of Gigahertz, and consisting of a network with four terminals formed by two mutually coupled lines so as to impart directivity to the coupling.
2. Description of the Related Art
Couplers for microwave signals are widely used, most often in the telecommunications applications. Known couplers typically consist of a network of two mutually coupled lines, each having two terminals. FIG. 1 shows the classical scheme of such a coupler, in which Lp and La represent the main line and the coupled line respectively, while Pi, Pu, Pa, Pd identify the inlet (Pi) terminal and the opposite (Pu) terminal on the line La. In general, the lines are disposed on a dielectric substrate SUB. The lines each have a width AP and a length L=1/4 .lambda., and are at a distance D from each other.
Still referring to FIG. 1, Wi represents the inlet power in the gate Pi, while Wa represents the outlet power of the coupler terminal Pa, with Wd representing the power at the output of the uncoupled terminal Pd, and Wr the power acted upon by the coupler and outgoing from the inlet terminal.
The coupler functions in accordance with following definitions:
Coupling Acc=Wi/Wa PA1 Directivity DIR=Wa/Wd PA1 Adaptation AD=Wi/Wr
With continued reference to FIG. 1, the parameters utilized to obtain the desired coupling (Acc) and simultaneously to have maximum values of directivity (DIR) and adaptation (AD) are the widths (AP) of lines Lp and La and the distance D between the lines.
The length L of the coupled zone (ZA) determines the frequency field in which the coupling (Acc) becomes flat and, as stated previously, L is set equal to a quarter of the wavelength of the central frequency Fc of the working band BAL. For values of L other than .lambda./4 the coupling is not flat, but the directivity and the adaptation are not significantly worsened. One of the inconveniences of the described conventional structure is that the directivity is rather low and does not reach satisfactory values no matter how the width AP of the coupled lines and the distance D between the lines are varied.
This becomes understandable if one considers that there are three magnitudes or factors which must be maintained (coupling Acc, directivity DIR and adaptation AD), whereas there are only two variable parameters (width AP of the lines and interline distance D). More specifically, reference is made to FIG. 2, in which the cross-section of the coupled zone is shown. The solid lines relate to the electric field E and determine the capacitance C of the coupling Acc, while the dashed lines relate to the magnetic field MA and determine the mutual inductive coupling M. The presence of air above the coupler dielectric substrate SUB decreases the capacitance of the coupling and prevents the capacitance from attaining the optimal value it should have with respect to the mutual inductance M.
The air present above the substrate is eliminated in "strip-line" couplers in which, as shown in FIG. 3, the coupled lines Lp, La are disposed between a double substrate SUB, SB1 and a double mass plane ma, ma1.
The coupler of FIG. 3 has improved characteristics and substantially solves the above-mentioned problem; however, it has the significant inconvenience of being embodied with the complex, expensive structure of a strip-line sandwich.